ext/json: objects depend on $(ruby_headers)

* ext/json/generator/depend: add build dependencies for json extension
  Patch by normalperson (Eric Wong) [Bug #9374] [ruby-core:59609]
* ext/json/parser/depend: ditto

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44514 b2dd03c8-39d4-4d8f-98ff-823fe69b080e